A compressor of a gas turbine engine comprises rotor components, including rotor blades and a rotor drum, and stator components, including stator vanes and a stator casing. The compressor is arranged about a rotational axis with a number of alternating rotor blade and stator vane stages as is well known and each stage comprises an aerofoil. The efficiency of the compressor is influenced by the running clearances or radial tip gap between its rotor and stator components. The radial gap or clearance between the rotor blades and stator casing and between the stator vanes and the rotor drum is set to be as small as possible to minimise over tip leakage of working gases, but sufficiently large to avoid significant rubbing that can damage components.
The pressure difference between a pressure side and a suction side of the compressor aerofoil causes the air to leak through the tip gap. The over-tip leakage flow results in large amounts of loss and blockage in the tip gap region of the compressor stage and which is detrimental to the stability and efficiency of compressors.
In a gas turbine engine the compressor is driven by a turbine. Like a compressor the turbine comprises a number of alternating rotor blade and stator vane stages. Hot working gas from a combustor impinges on the turbine blades, which are mounted to a turbine rotor disc, and forces the rotor disc to rotate thereby driving the compressor. The compressor blades are forced to rotate and draw in air to the engine and compress it. Thus there is a fundamental difference between turbines and compressors, with turbines blades extracting energy from the hot working gases while compressors impart energy to the air stream. From the upstream side to the downstream side of each turbine blade stage the pressure of the working gas flow decreases as work is extracted from the working gas, whereas for a compressor stage the pressure increases across each stage as work is input to the air stream.
Reduction of over-tip leakage in turbines has been addressed in a number of blade tip configurations including winglets. Essentially, a winglet is an overhang extending from the tip of the turbine blade in a pitch-wise direction to overhang a pressure and/or suction surface of the blade. Turbine winglets are designed to reduce the pressure difference from the pressure side to the suction side and over the blade tip. This pressure difference over the blade tip will be referred to as tip loading. These turbine blade winglets are specifically designed to accommodate the drop in working gas pressure that occurs from the leading edge to the trailing edge of the blade. Examples of these turbine winglets include EP 2 093 378, US 2010/0135813, U.S. Pat. No. 7,632,062, 8,414,265 and US 2005/0232771. Turbine blade winglets can be designed to minimize heat transfer into the blade. In EP 2 725 195 A1 the winglet is designed to move an over-tip leakage vortex away from the suction surface of the blade thereby preventing additional hot gases impinging on and increasing the temperature of the blade material.
The present invention relates to the configuration of a winglet applicable only to compressor aerofoils. The impact of winglets on compressor aerofoils and turbines blades is fundamentally different in nature because the efficiency of compressors is limited by corner separations while for turbines winglets are designed to reduce tip loading and reducing heat transfer. The applicant has found that application of a turbine winglet to a compressor blade can actually reduce efficiency by increasing the size of or causing a corner separation near the aerofoil tip. Thus the present invention addresses not only a reduction of over-tip leakage mass flow, but importantly addresses separations unique to compressor aerofoils.